Use str.replace in make_gaps_ambiguous

[groutr]

Description of proposed changes

This PR switches out the numpy based implementation for one that uses str.replace. Not only is it significantly faster, but it is also more memory efficient.

In [221]: %timeit make_gaps_ambiguous(a)
1.24 ms ± 27.1 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

In [222]: %timeit str_replace(a)
7.78 µs ± 283 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

Testing

Ran the two implementations on the same input (a case with no '-' characters and a case with '-' characters) and observed the same output.

[huddlej]

Thanks, @groutr and @Zsailer!